An electrophotographic photosensitive member having an undercoat layer and a photosensitive layer formed in this order on a support has been used in electrophotographic apparatus.
In some known technologies, the undercoat layer contains metal oxide particles for improved conductivity. PTL 1 describes a technology in which the undercoat layer contains titanium oxide particles coated with phosphorus- or tungsten-doped tin oxide. PTL 2 describes a technology in which the undercoat layer contains aluminum-doped zinc oxide particles. PTL 3 describes a technology in which the undercoat layer contains titanium oxide particles coated with oxygen-deficient tin oxide. PTL 4 discloses a technology in which the undercoat layer contains barium sulfate particles coated with titanium oxide. These known electrophotographic photosensitive members, in which the undercoat layer contains metal oxide particles, satisfy the current image quality requirements.
In recent years, electrophotographic apparatus have been getting faster and faster (in terms of process speed or cycle speed) and it has been demanded that an electrophotographic photosensitive member perform better in repeated use.
The inventors found through research that the electrophotographic photosensitive members described in the above literature, having an undercoat layer that contains metal oxide particles, become more likely to have the following problems with increasing process speed of the electrophotographic apparatus. More specifically, they have room for improvement because repeated image formation with them under low-temperature and low-humidity conditions can cause many of output images to have streaks caused by charge (hereinafter, charge streaks). Charge streaks are streak-like image defects extending perpendicular to the longitudinal direction of charge of a surface-charged electrophotographic photosensitive member, and they occur as a result of the electrophotographic photosensitive member experiencing a decrease in the uniformity of its surface potential (charge nonuniformity). Charge streaks are particularly common when a half-tone image is output.